Mammalian neurosecretory neurons are excellent models for understanding the relationship between spike discharge patterns and hormone/transmitter release in the central nervous system. Neurosecretory neurons adopt a bursting pattern that promotes maximally efficient stimulus-secretion coupling at the nerve terminal. The phasic bursting activity in vasopressin neurons is exemplary of this pattern, and relatively unique in that it can be studied in vitro, as it results largely from intrinsic membrane properties. The global hypothesis of this proposal is that phasic bursting per se can be explained largely by the understanding of one primary current, its modulation by Ca++, and its auto-regulation by dynorphin through kappa opiate receptors. The Specific Aims are: Aim 1) Determine the ion species, underlying conductance change, and Ca++-dependence of the current underlying the DAP (IDAP). Experimentally, we will test several predictions from a powerful computational model simulating phasic activity developed during the past grant period. We predict that the plateau potential underlying bursts results from a Ca++ dependent inhibition of a K+ leak current, such that this current attains voltage-dependence in elevated [Ca++]i. Aim 2) Determine the mechanism of the auto-regulatory inhibition of VP neurons via kappa opiate receptors. We predict that dynorphin, released locally during a burst from vasopressin neurons, shifts the Ca++ sensitivity of a K+ leak current rightward, raising its threshold, and terminating the burst. Aim 3) Determine the role of auto-regulation in the variability of phasic bursting expression in vitro. We predict that phasic bursting is correlated with the ability of dynorphin to regulate the DAP and burst length and, complementarily, that deficits in phasic bursting activity are due to deficits in auto-regulation. A corollary is that phasic bursting may be related to the degree of dendritic arbor present. Understanding the origin of phasic bursting is critical to understanding how VP release is controlled in both physiological and pathophysiological conditions. VP release is critical to normal water balance and cardiovascular regulation.